Ballast circuits for discharge lamps

ABSTRACT

A ballast circuit for a mercury halide discharge lamp comprises a bridge rectifier providing a path between input and output terminals of the ballast circuit, and includes at least one electronic voltage doubling or tripling circuit providing a second path between the input and output terminals. The voltage doubling or tripling circuit contains a capacitance which is sufficiently high to effect during starting ionization of the starting gas and establish a state of conduction in the discharge lamp while having a sufficiently high impedance to be substantially ineffective during normal running of the lamp.

United States Patent Major et al.

[ 1 Dec. 24, 1974 BALLAST CIRCUITS FOR DISCHARGE LAMPS Inventors: Peter Colin Major; Kenneth Whittier Ogden, both of London, England Assignee: Thorn Electrical Industries Limited,

London, England Filed: Feb. 15, 1973 Appl. No.: 332,717

Foreign Application Priority Data Feb. 16, 1972 Great Britain 7257/72 US. Cl 315/170, 315/171, 315/173, 315/176, 315/205, 315/207, 3l5/D1G. 5

Int. Cl. 1105b 41/23 Field of Search... 315/101, 170,171,173-175, 315/176, 205, 207, DIG. 5

References Cited UNITED STATES PATENTS 6/1959 Popa 315/205 3,170,084 2/1965 Retzer 315/205 X 3,354,379 11/1967 Swain et all 315/D1G. 5

3,676,735 7/1972 Ruehe t 315/207 3,679,934 7/1972 Dukes....... 315/171 X 3,758,815 9/1973 Paget 315/176 X Primary Examiner-James B. Mullins Attorney, Agent, or Firm-Kemon, Palmer & Estabrook 5 7 ABSTRACT 4 Claims, 10 Drawing Figures o H l Patented Dec. 24, 1974 6 Sheets-Sheet 1 Patented Dec. 24, 1974 3,857,063

6 Sheets-Sheet 2 49 58 54 SUAI. r H l PPL I 5:} H J: 40 52 80$ 1 m I 8 46 J I l 33 86 i L J Patented Dec. 24, 1974 3,857,063

6 Sheets-Sheet 3 AL H 0 SUPPLY 66 68 FIG. 6 82 I c. 60 H p sumr (L 66 as Patented Dec. 24, 1974 3,857,063

6 Sheets-Sheet 4 Patented Dec. 24, 1974 3,857,063

6 Sheets-Sheet 5 FIG] l/ He. 8. T

1 BALLAST CIRCUITS FOR DISCHARGE LAMPS BACKGROUND OF THE INVENTION This invention relates to improvements in ballast circuits for discharge lamps, particularly, though not exclusively for use with mercury halide discharge lamps.

There exist many types of ballast for discharge lamps of varying degrees of effectiveness and complexity. Mercury halide lamps have however proved to be particularly difficult to ballast because of the very high pre-strike voltages required to start them. This is all the more difficult when it is desired to run the lamp on direct current. US. patent application Ser. No. 279,445 filed Aug. 10, 1972, now Patent. No. 3,787,751, discloses one type of ballast providing a dc output from an ac. source which produces a peak open-circuit pre-strike voltage of /2 times the r.m.s. supply voltage. This is sufficient to strike a high pressure mercury vapour discharge lamp, but insufficient to strike a mercury halide discharge lamp. A similar problem will arise in a simpler ballast circuit comprising a series inductor followed by a bridge rectifier.

Other attempts to produce workable circuits have required transformers, which are bulky and expensive, or mechanical contacts which are unreliable.

SUMMARY OF THE INVENTION In accordance with this invention there is provided a ballast circuit for a discharge lamp, comprising input terminals for connection to an a.c. source of supply, output terminals for supplying direct current to a discharge lamp, a bridge rectifier providing a first path between the input terminals and the output terminals, and an electronic voltage increasing circuit providing a second path between the input and output terminals, the voltage increasing circuit containing a sufficiently high capacitance to be effective during starting of a discharge-lamp and a sufficiently high impedance to be substantially ineffective during normal running of the lamp.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a basic ballast circuit embodying the invention;

FIG. 2 is a diagram showing lamp waveforms associated with the ballast of FIG. 1',

FIGS. 3, 4 and 6 show modifications of the basic circuit of FIG. 1;

FIG. is a diagram showing lamp waveforms associated with the ballast of FIG.-4;

FIGS. 7 and 8 show preferred ballasts embodying the invention; and

FIGS. 9 and 10 show further modifications of the circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a ballast circuit adapted to provide sufficient pre-strike voltage to enable it to be used with mercury halide discharge lamps. The ballast includes two input terminals 60 to one of which is connected an inductor 62. A bridge rectifier 64 comprising diodes 66, 68, 70 and 72 is connected to the inductor 62 and the other input terminal and provides an output to a lamp 74.

The ballast also includes a voltage doubler circuit 80 which is of itself of conventional type and when supplied with a voltage Vsinwt produces a dc output of 2V. The voltage doubler circuit consists of a capacitor 82 and diode 84 connected in series across its inputs, and a diode 86 and capacitor 88 connected across the diode 84. The output is taken across the capacitor 88 and is connected to the appropriate end of the lamp 74, in this case that end which is connected to the diodes 68 and 72.

The peak open-circuit pre-strike voltage across the lamp 74 is thus 3V, this being shown in FIG. 2. Before the lamp strikes, the voltage across the capacitor 88,

and hence at one end of the lamp, is 2V, while the voltage at the other end of the lamp is that across diode and varies between 0 and -V volts. The resultant voltage across the lamp varies from 2V to a peak of 3V. Once the lamp has struck the voltage doubler circuit has no significant effect, provided that the values of capacitors 82 and 88 are such that their reactances are large compared with the reactance of inductor 62. For a 400 watt mercury iodide lamp suitable values for the capacitors 82 and 88 would be of the order of 0.5 microfarads. The capacitors 82 and 88 must be sufficiently large to be able to supply the required current at starting but sufficiently small to have no real effect on the operation of the circuit when the lamp is running normally. That is to say, they must present a high impedance to the lamp current in normal operation. The choke 62 is of conventional type.

Thus during starting of the lamp, when the lamp presents a high impedance, the voltage doubler circuit 80 is effective to add to the voltage provided by the bridge rectifier 64. During normal running the lamp is'supplied only through the bridge rectifier 64, and the voltage doubler 80 plays no effective part in the operation of the ballast. It will be seen therefore that lamp current does not in normal operation pass through the voltage doubler circuit.

During starting the voltage doubler circuit has two functions, first to provide sufficient voltage across the lamp to ionise the starting gas, and second to pass sufficient energy through the gas and lamp electrodes to establish a state of conduction in the discharge which can be maintained by the lower supply voltage. This second phase of starting is the result of capacitor discharge.

A number of simple modifications can be made to the circuit of FIG. 1 without affecting its operation. Instead of taking the input to the voltage doubler circuit 80 across the terminals 60, it can be taken after the series inductor 62. Also, the inputs of the circuit 80 can be reversed. Both these modifications are shown in FIG.

3, in which the voltage doubler circuit 80 is connected to the ballast of FIG. 5 of our aforementioned application, to which reference should be made for a detailed description of the bridge rectifier. Briefly, the bridge comprises two bridge circuits with certain circuit elements in common. The first bridge consists of inductor 50, and diodes 54, 58, 46 and 48, and the second bridge consists of capacitor 49, and diodes 52, 56, 46 and 48. The outputs of the bridges are effectively added, and in this way a high power factor can be obtained. The voltage doubler circuit 80 itself is identical to that of FIG.

1. With this circuit, it is also possible to take one input to the voltage doubler circuit from after the capacitor 49 rather than after the inductor 50.

The polarity of the diodes84 and 86 may be reversed. In this case, the output of the voltage doubler circuit 80 in FIG. 1 is connected to the other terminal of the lamp 74. All these modifications produce an open-circuit pre-strike voltage of the form shown in FIG. 2.

Referring again to FIG. 1, it will be seen that the diodes 84 and 86 are effectively in parallel with diode 72. Thus diode 72 can be removed without affecting the operation of the circuit, as shown in FIG. 4. Also in FIG. 4, the capacitor 88 has been removed. The opencircuit voltage V8436 across the two diodes 84 and 86 is now sinusoidal with a mean of +V volts and peak of +2V volts, as shown in FIG. 5. The voltage V across diode 70 is as before, and the open-circuit pre-strike voltage V across the lamp is thus as shown in FIG. 5, having a peak of 2V and minimum of V volts. This peak voltage is still sufficient to strike-a mercury iodide discharge lamp at a supply voltage of 200 250 volts r.m.s.

Further developing the circuit of FIG. 4, it will be seen that the diode 86 no longer serves any useful circuit function, as it is effectively in series with diode 70, and can be replaced by a direction connection. This gives rise to the circuit shown in FIG. 6, here again the peak open-circuit pre-strike voltage across the lamp is twice the r.m.s. supply voltage.

The circuit of FIG. 6 can benefit from the addition of a resistor in series with the capacitor 82 to prevent extinction of the lamp during run-up. Such a resistor is included in FIG. 7, which shows a preferred embodiment of the invention in which the capacitor 82 is connected through resistor 90 to the other one of the input terminals.

In the circuit of FIG. 7, assuming the lamp is the same 400 watt mercury iodide lamp referred to earlier, a suitable value for capacitor 82 is 2 microfarads, and for resistor 90 is l kilohm. The resistor 90 reduces the current through the capacitor 82 when the voltage reverses across diode 70, thus preventing the capacitor 82 drawing so much current that the lamp would extinguish.

' g The circuit of FIG. 8 is based on FIG. 7 and uses the same component values but is adapted to provide an open-circuit voltage across the lamp of Vsinwt 2V resulting in a peak pre-strike lamp voltage of 3V. To this end the circuit includes two capacitors 82 and 82" connected in series with respective resistors 90' and 92" to the two ends of the lamp 74.

FIG. 8 is in fact based on the circuit of FIG. 1 with a further voltage doubler of opposite polarity connected between the other supply input terminal 60 and the other end of the lamp 74. A circuit which included two voltage doubler's of the type 80 shown in FIG. 1 would produce a voltage across the lamp 74 of 4V Vsinwt, giving a peak of 5V.

Considering again the circuit of FIG. 1, but with the capacitor 82 connected to the lamp side of the choke 62, the circuit performance is in no way affected if the connection from the cathodes of diodes 68 and 72 is made to the anode of the diode 86, instead of to its cathode, as shown in FIG. 9. Before the lamp strikes, capacitors 82 and 88 are charged to V and 2V respectively, and the pre-strike voltage has the form shown in FIG. 2. This can be increased to a steady 3V by the addition of a further capacitor 94 in parallel with diode 70, which is charged to the peak supply voltage V through diode 66, as shown in FIG. 10. Also shown in this figure are resistors 96 and 98 in series with capacitors 88 and 94 respectively to prevent extinction of the lamp during run-up.

It will thus be appreciated that the circuits illustrated enable mercury halide discharge lamps to be operated on d.c. circuits with a high starting voltage.

It should be noted that many features of the various embodiments and modifications described above and in our earlier application can be employed in different combinations and arrangements to similar effect.

We claim:

1. A transformerless ballast circuit for a metal halide discharge lamp, comprising:

two input terminals for connection to an a.c. source of supply;

two output terminals for supplying direct current to the metal halide discharge lamp;

a bridge rectifier coupling said two input terminals and said two output terminals; and

a capacitor and resistor connected in series circuit in parallel with said bridge circuit and coupling one of said input terminals and one of said output terminals, said series circuit presenting a sufficiently high capacitance to be effective during starting of said discharge lamp and a sufficiently high impedance to be substantially ineffective during run-up and normal running of said lamp.

2. A ballast circuit according to claim 1, including a second capacitor and a second resistor connected in second series circuit coupling the other of said input terminals and the other of said output terminals, said second series circuit also presenting a sufficiently high capacitance to be effective during starting of said discharge lamp and a sufficiently high impedance to be substantially ineffective during run-up and normal running of said lamp.

3. A ballast circuit according to claim 1, wherein the capacitor has a capacitance of about 2 microfarads and the resistor has a resistance of about I kilohm.

4. A ballast circuit according to claim 1, including a choke between one of said input terminals and said bridge rectifier. 

1. A transformerless ballast circuit for a metal halide discharge lamp, comprising: two input terminals for connection to an a.c. source of supply; two output terminals for supplying direct current to the metal halide discharge lamp; a bridge rectifier coupling said two input terminals and said two output terminals; and a capacitor and resistor connected in series circuit in parallel with said bridge circuit and coupling one of said input terminals and one of said output terminals, said series circuit presenting a sufficiently high capacitance to be effective during starting of said discharge lamp and a sufficiently high impedance to be substantially ineffective during run-up and normal running of said lamp.
 2. A ballast circuit according to claim 1, including a second capacitor and a second resistor connected in second series circuit coupling the other of said input terminals and the other of said output terminals, said second series circuit also presenting a sufficiently high capacitance to be effective during starting of said discharge lamp and a sufficiently high impedance to be substantially ineffective during run-up and normal running of said lamp.
 3. A ballast circuit according to claim 1, wherein the capacitor has a capacitance of about 2 microfarads and the resistor has a resistance of about 1 kilohm.
 4. A ballast circuit according to claim 1, including a choke between one of said input terminals and said bridge rectifier. 